1. Field of the Invention
The present invention relates to a tracking servo apparatus for use in an optical disk player.
2. Description of Background Information
Because of the so-called eccentricity which is a displacement of a center of a disk itself from a center of a track formed on the disk, the relative position of an irradiation spot which is a position for reading information by a pick-up is always displaced from that of a track in the radial direction of the disk. To apparently eliminate the relative positional displacement in the radial direction of a disk, a tracking servo apparatus is used so that a pick-up can always follow a track accurately.
The tracking servo apparatus generally has means for generating a tracking error signal having a level and a polarity each corresponding to a distance between a pick-up and a track as well as to the direction thereof, and moves and controls a position of an irradiation spot by a pick-up in the radial direction of a disk by way of forming a tracking servo loop by driving a tracking actuator inside the pick-up according to the tracking error signal. If an eccentric disk is rotated in a case where, for instance, an operation of a disk player is started, a traversing frequency of a track viewed from the pick-up may surpass at maximum 10 KHz. In contrast, a bandwidth of tracking servo is in a range from 500 Hz to around 2.5 KHz, and even if a bandwidth of tracking servo is switched between the drawing mode in which a tracking servo loop is closed and the tracing mode, drawing is difficult when a traversing frequency of the track is high. There is a method of monitoring a tracking error signal frequency and drawing into tracking servo control when the frequency becomes lower, and this method is based on the fact that a traversing frequency becomes lower 2 times for each one rotation of a disk. However, in this method, a long time is required before drawing is started, and drawing is delayed disadvantageously.
To solve this problem, a braking operation is often carried out to lower a relative speed by moving an irradiation stop in the traverse direction for the purpose to draw a tracking servo rapidly even when the frequency of traversing the track is high.
In a conventional type of tracking servo apparatus, an envelope component of an RF signal which is a read signal read by a pick-up from a disk is obtained, the envelope component and the tracking error signal are converted to binary values respectively, the envelope component converted to a binary value is sampled at an edge of the tracking signal also converted to a binary value and the sampled envelope component is maintained until the next edge to generate a tracking gate signal. Switching of a tracking servo loop is controlled according to the tracking gate signal, and either one of the polarity sections of the tracking error signal is selected, the selected one polarity section give effects to a tracking actuator as a brake signal corresponding to the traversing direction of an irradiation spot. With the braking operation as described above, the tracking servo loop is set to a closed state when a frequency of the tracking servo signal goes down below a frequency allowing drawing, and tracking servo control is executed. A tracking servo apparatus having the configuration as described above is disclosed, for instance, in Japanese Patent Kokoku No. 63-4271 or in Japanese Patent Kokoku No. 1-54794.
However, when a double density track disk is used as an optical disk, an adjacent track is included in an irradiation spot, a level of an RF signal becomes substantially equal regardless of whether an irradiation spot is on a ON track or on an OFF track, and an envelope component of the RF signal little changes, which makes it impossible to execute a braking operation using an envelop component of an RF signal.